In a wireless communications system such as a Long Term Evolution (LTE for short) system, a terminal is usually in two states: a radio resource control (Radio Resource Control, RRC) connected mode and an idle mode. Mobility of the terminal in both the two states is controlled by a network device such as a base station in the LTE system.
When the terminal is in the RRC connected mode and moves between cells, the network device determines whether the terminal needs to be handed over to a new cell, makes preparations for performing cell handover between a source cell from which the terminal is handed over and a target cell to which the terminal is handed over, and then instructs the terminal to access the target cell.
As shown in FIG. 1, a source base station sends a measurement configuration message to a terminal, and controls the terminal to perform wireless measurement on a source cell and a neighboring cell. In a process in which the terminal moves from the source cell to a target cell, when a condition indicated by the measurement configuration message is met, the terminal sends a measurement report to the source base station. The source base station performs handover decision, sends a handover request to a target base station of the target cell; and after receiving a handover request acknowledgement from the target base station, sends a handover command to the terminal, so as to instruct the terminal to be handed over from the source cell to the target cell.
In the RRC connected mode, the terminal cannot initiate a new RRC connection establishment process to access a new cell. Instead, the network device such as a base station or a core network device such as a mobility management entity (Mobility Management Entity, MME) needs to determine whether a new cell needs to be added for the terminal, so as to improve a maximum peak rate of the terminal.
In the idle mode, if the terminal needs to enter the RRC connected mode to perform data transmission, the terminal can access only a cell on which the terminal currently camps, and cannot access another cell.
When the terminal is in the idle mode and moves between cells, the terminal determines, according to a cell reselection rule in the cell on which the terminal currently camps and a cell reselection rule provided by the last cell after the terminal enters the RRC connected mode, whether to perform cell reselection, and which cell is to be reselected. These cell reselection rules are also specified by the network device. The terminal strictly performs cell reselection according to the cell reselection rule specified by the network device.
In conclusion, in a current wireless communications system such as the LTE system, both RRC state transition and mobility management of the terminal, for example, a cell handover process, a cell reselection process, and a cell access process, need to be performed under the control of the network device. The terminal strictly performs an operation according to an instruction of the network device.
In the foregoing manner in which the network device strictly controls the terminal, although it is convenient to centrally manage the terminal by the network device, network implementation is complex, and deployment and maintenance are not easy to perform.